Dietary fat is one of the three energy-providing macronutrients, alongside protein and carbohydrates. At about 9 kcal per gram, it is the most energy-dense. This entry is about fat in food, not body fat.
What you need it for
Fat is needed for hormone production and for absorbing fat-soluble vitamins. That is why you should not keep it very low for long, not even in a cut. Beyond that there is a lot of room: how you split your calories between fat and carbs after protein is largely a matter of preference and fullness.
How much makes sense
The reference values of the US Institute of Medicine (2005) set 20 to 35 percent of energy from fat for adults. A rule of thumb is not to stay below about 20 percent for long. For contest preparation, Helms et al. (2014) give 15 to 30 percent, which is an exceptional situation with a hard diet. For the off-season, Iraki et al. (2019) recommend 0.5 to 1.5 g per kg of body weight.
Example: With a calorie target of 2,600 kcal and 25 percent fat, that is 650 kcal. Divided by 9, you get about 72 g of fat a day. These numbers are a worked example.
Fat and calories
Because fat provides more than twice as much energy per gram as protein and carbs, it adds up quickly. Oil, nuts, cheese and fatty meat pack a lot of energy into little volume. In a bulk, that can help you reach your surplus. In a cut, it makes staying full harder, which is why high-volume foods with little energy per bite are useful there. Whether the fat you eat makes you fatter depends, as with any nutrient, on energy balance.